Geodetic instruments, such as theodolites or transits, are normally mounted upon tripods or similar supports during actual use in the field. In order that the spindle of an instrument alidade may be referenced precisely to the vertical datum, means are usually provided, either as an integral part of the geodetic instrument or as a separate fixture, by which the base of the instrument may be adjusted in two orthogonal planes. Such leveling means are usually situated between the head plate of the tripod and the base plate of the geodetic instrument and comprises a plurality of threaded adjusting legs by means of which the instrument may be tilted with respect to the tripod head in either of the two planes in order to achieve a level datum condition.
Previous leveling means for geodetic instruments have comprised the four-screw combination, such as may be seen in U.S. Pat. No. 2,746,155, or the three-screw system generally described in U.S. Pat. No. 3,417,394. Substantially equivalent to the three-screw leveling means is that employing two screws and a third cooperating stationary post.
Adjustments utilizing the four-screw system require simultaneous manipulation of diametrically opposed screws in order to effect leveling movement in each of the two planes. As a result, it is not uncommon for such four-screw systems to suffer from errors in manipulation such that one or more of the adjustment screws is forcibly strained against movement of another with resulting permanent damage to the leveling system. While the three-screw system for leveling a geodetic instrument does not suffer in a similar manner, it does have a disadvantage of azimuthal instability unless highly precise adjusting screws are employed. Also, due to the fact that three-point systems comprise a single mobile platform, cross-coupling is an inherent drawback. That is to say, a desired adjusting movement in one of the two leveling planes is not isolated to that plane, rather some vector of the movement is translated into movement in the other plane, resulting in continuous misadjustment or, at best, a prolonged adjusting procedure.
The instrument leveling means of the present invention comprises two independently mobile platforms, the tilting movement of each of which is controlled by a single separate adjusting screw. The freedom of movement of the platforms and the isolation of the mobility of one from the other eliminate the noted major disadvantages of previous types of leveling systems. The construction of the present leveling platform, or tribrach, is such that the system as a whole maintains a particularly low profile which in higher order geodetic instruments eliminates misreadings which often result from the effects of wind upon the supported instrument.